1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting element, in particular, a nitride semiconductor light emitting element having a plurality of peaks in its emission spectrum.
2. Discussion of the Related Art
In recent years, semiconductor light emitting elements capable of emitting light in the ultraviolet region have been put into practical use, which are also used in an exposure apparatus, a disinfection apparatus, and the like. However, in the cases where a semiconductor light emitting element is used in a ultraviolet irradiating apparatus for curing a material, with a larger thickness of the irradiated material, insufficient curing of a deeper portions may result. The reasons include that the ultraviolet irradiation cannot penetrate into a deeper portion of the irradiated material. In order to allow light to penetrate deeper into the irradiated material from the surface to a deeper portion, it is efficient that the ultraviolet irradiating apparatus has a plurality of emission peak wavelengths. To obtain such an apparatus, a plurality of semiconductor light emitting elements having different peak wavelengths or a semiconductor light emitting element having a plurality of active layers having different compositions may be employed (see Japanese Laid-Open Patent Publication No. 2007-305703).
A semiconductor light emitting element having a plurality of well layers with varying compositions may be intended to provide white light with a single semiconductor light emitting element. Specific examples thereof include a structure having well layers of different emission wavelengths stacked in order of wavelength, and a structure having a repetitive stack of such structures (see Japanese Laid-Open Patent Publications Nos. H10-22525, 2001-28458, and 2004-128443).
Other examples of varying compositions in the active layer include a quantum well structure having a strain compensating layer with a larger band gap energy than that of a barrier layer interposed between the barrier layer and a well layer (see Japanese Laid-Open Patent Publication No. 2004-87763).
However, in the semiconductor light emitting element in which well layers of different emission wavelengths are simply stacked in order of wavelength, it is difficult to produce an efficient and stable ratio of emission peak intensity. For example, in a case where a well layer of longer wavelength emission having a smaller band gap energy is disposed closer to the n-type semiconductor layer side than a well layer of shorter wavelength emission having a larger band gap energy, the luminous efficiency in longer wavelength decreases, so that stable ratio of emission peak intensity is difficult to obtain. Such a nitride semiconductor light emitting element having unstable ratio of emission peak intensity is unsuitable as the light source for curing a resin or an ink.
On the other hand, if a well layer of longer wavelength emission is disposed closer to the p-type nitride semiconductor layer side than a well layer of shorter wavelength emission, emission from the well layer of longer wavelength emission is extremely facilitated, and in some cases, emission from the well layer of shorter wavelength emission may not occur.